Add initial unit tests

Related to gitlab-org/gitlab#415539 (closed)

This MR does the following:

• introduces an initial set of specs to test the resources created match the expected resources & the if/else blocks in all templates.
• includes a helm_template.rb file from the charts project with one new method added
• renames the two ConfigMap resources so they follow the naming pattern used by the rest of the resources (gitlab-zoekt or gitlab-zoekt-gateway)
• renames the external gateway ConfigMap name so it's more clear what the resource does
• adds rspec to CI
• adds rspec to lefthook

How to test

bundle install
bundle exec rspec